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Human skin cells are a frequently reported biogenic particulate
of indoor environments. They have been identified as odor
producers, as well as fuel for microbial entities, beyond being a
common contaminant of buildings. Over 99 percent of samples
collected from both air and surfaces of indoor environments are
reported positive for these entities by utilizing microscopic
techniques. Non-culture based techniques reveal that it is one of
the dominating bio-components in indoor environments, besides other
abiogenic and biogenic particulates. Amelioration of these
particles provides an enhanced experience of indoor air quality.
Management of these building pollutants provides a significant
improvement on health, energy and comfort.
According to the United States Environmental Protection Agency
(EPA), Americans spend up to 90 percent of their time indoors (EPA
1989), where contaminants in bioaerosols are generally at higher
levels than those found in outdoor air (EPA 1987). Frequently, the
duration of exposure to such contaminants is also greater indoors
than out. It is estimated that more than 30 percent of buildings in
the United States and Western Europe have moisture problems serious
enough to promote microbial contamination of indoor air. Exposure
to high levels of indoor moisture is associated with upper
respiratory symptoms, including higher incidences of coughing,
wheezing, and asthma in sensitized persons. Additional case
studies, cluster investigations, and clinical experience associate
other health complaints with living and working in damp buildings
where mold and bacteria grow.
Healthcare professionals face a challenge due to symptoms being
common and associated with many different disorders. Medical
conditions associated with exposure to viruses, bacteria, or fungi
include infectious diseases, respiratory disorders such as
bronchitis and asthma, and other allergic, inflammatory, and toxic
responses. In some cases, evidence links these disorders to
exposure of bioaerosols. However, many times it is hard to link
environmental risk factors in the absence of information on the
causal elements. Skin cells are identified as one such particulate
that may have adverse comfort and health implications on the
occupants, which reduces productivity and quality of life.
Skin cells, or epithelial cells, are a frequently reported,
airborne, biological component in indoor environments. Skin is the
largest organ of the human body and it serves as a protective layer
as well as a natural sensor. Cells grow and divide in the basement
membrane causing newly developed cells to be pushed up into the
epidermis. The supply of blood and other nutrients eventually stop
reaching these cells and they slowly begin dying. The new skin
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cells replace the sloughing ones. These cells consist of
keratinocytes, melanocytes, Merkel cells and Langerhans’ cells and
they are macroscopic and microscopic in nature. The size range of
microscopic particles may vary in size from 5 micrometers (commonly
10-100 micrometers) and above. Biochemically, they contain squalene
and cholesterol in addition to water.
These cells are often reported as part of dust microflora in
indoor environments, both in air as well as from surfaces. The skin
cells are often overlooked in importance as a significant agent
that may impact the quality of living space. Abiogenic or biogenic
particulates such as bacteria, fiber, fiberglass, mold spores/fugal
constituents, pollen grains, insect bio-detritus, inorganic
particulates (that appear opaque under light microscopy), and
others are some commonly encountered entities from closed
environments that can be linked to health and hygiene related
issues. However, it is estimated that 75-90 percent of household
dust is made up of skin cells. A large portion of these bodies in
indoor dust flora mainly depend on occupancy, humidity level and
overall dermal health of occupants. It is estimated that humans
shed 500 million cells a day, and drop off 30 to 90 mg of skin
flakes every hour (Weschler et al. 2011). Subsequently, these
particles of biological origin are mixed with dust and spread in
and around indoor environments both on the surface as well as in
the ambient air. These fragments also fuel and support the growth
of many microorganisms including bacteria, fungi, mites, etc. Some
common bacteria associated with human skin cells are
Staphylococcus, Micrococcus, Corynebacterium, Propionibacterium,
Brevibacterium, Acinetobacter, etc., whereas, Trichophyton, Candida
and Malassezia are amongst some common fungal organisms. Also,
Demodex mites such as Demodex folliculorum and Demodex brevis may
feed on epithelial cells.
A new study, published in the American Chemical Society’s
journal, Environmental Science & Technology, concludes that oil
in those skin cells makes a small contribution to reducing indoor
air pollution. However, a number of other study suggests that house
dust can trigger allergic reactions and harbor a number of
microorganisms that can influence health and hygiene. A team of
researchers led by Dr. Lai Kaman reveals that human skin flakes can
lead to a bad smell in air-conditioning systems. They have reported
that skin squalene shed from the human body can contribute to
ammonia and volatile fatty acid production by bacteria colonizing
air-cooling units. In evaluation of indoor environment quality,
these components of nuisance dust are significant especially in
cases where unexplained odors are causing potential issues leading
to discomfort and low productivity.
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Utilizing an on-call basis for testing, 5,197 air samples were
collected from approximately 2,359 buildings across 48 states in
the US and territories along with one foreign country between the
periods of April 2007 to December 2017. 14,056 Bio-Scan surface
samples were collected from over 8,000 buildings located across 53
states in the US and its territories including one foreign nation
during a period ranging from December 2003 to December 2017.
Surface samples were collected utilizing Bio-Scan 400 (a sticky
tape imprint method), whereas, air samples were taken by drawing 15
liters of air for three minutes over a glass slip coated with an
adhesive by spore trap method at 3’ to 4’ from the surface level.
The spore trap method operates upon the principle of inertial
impaction.
Samples collected using tape imprint methods are prepared for
microscopic examination by transferring the entire Bio-Scan on top
of a microscope glass slide (Premiere Premium Charged Fine Ground
Edge 75 x 25 x 1mm) facing the specimen side toward the glass
surface. Spore trap samples are processed by transferring the
specimen collected on the microscope glass slide. One drop of
lactophenol cotton blue stain is placed in the center of a clean,
sterilized and labeled microscopic slide. The glass slip with the
collected sample is removed carefully from the cassette and placed
directly onto the drop of staining solution (the collection surface
of the glass slip down, facing the stain) with the help of tweezers
so that the stain disperses evenly. These slides were allowed to
sit for about 5-10 minute prior to analysis. Qualitative
identification of skin cells is based on reference slide
comparisons.
A Bio-Scan has 400 squares that are 1 mm2 in surface area each.
Twenty-five randomly chosen squares representing all four zones and
the center of the Bio-Scan are selected for microscopic evaluation.
The primary microscopic evaluation of the collected samples is
undertaken using 100x magnification (10x10), whereas,
identification and quantification of skin cells is performed under
400x magnification (10x40).
Microscopic examination of the spore trap collected samples is
initiated by scanning the entire trace for skin cells under 100x
magnification (10x10). The counting and affirmation of skin cell
particles in the specimen are completed by using 400x magnification
(10x40).
Statistical analysis of all the above collected data is computed
using a Computer Assisted Air Management Program (a proprietary
Laboratory Information Management System, i.e., Paradox, for Pure
Air Control Services, Inc.) and presented in Table-1 & 2.
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lists the total number of air samples considered for the study.
The table includes positive samples and negative samples as well as
the mean, median, and mode values of skin cells trapped from the
ambient air from specified indoor sites. Concentration of skin
cells at the 67th percentile is also calculated and recorded into
this table.
includes the frequency and concentration of surface particulates
from indoor environments. Data is recorded for low and high values
along with middle, mean, and most commonly observed concentrations
of skin cells in terms of counts/cm2. Concentration of surface
particulates from indoor environments at the 67th percentile is
also included.
shows the frequency of skin cells identified both from air and
surface samples in terms of total samples considered for this
study.
illustrates the average concentration of skin cells and other
particulates identified from air and surface samples collected from
indoor environments using microscopic techniques.
shows skin cell fragments in environmental samples.
Particle Type Total
Samples
Positive
Samples
Negativ
e
Samples
Concentration (Counts/m3)
Low
Value
High
Value
Median
Value
Mean
Value
Mode
Value
67th
percentile
Skin Cells 5197 5156 41 22 171822 2800 4883 66 4940
Opaque
Particles 5197 5186 11 22 204000
0 14100 28069 1730 24000
Insect
Biodetritus 5197 369 4828 22 1980 0 4 0 0
Fibers 5197 4720 477 22 66599 155 309 0 266
Pollen Grains 5197 1263 3934 22 46521 0 40 0 0
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Fungal Spores 5197 4615 582 22 833000 198 3573 0 488
Others 5197 5163 34 22 2340000 2030 6654 622 3920
Particle Type Total
Samples
Positive
Samples
Negativ
e
Samples
Concentration (Counts/cm2)
Low
Value
High
Value
Median
Value
Mean
Value
Mode
Value
67th
percentile
Skin Cells 14056 13073 983 4 102500 160 949 0 356
Opaque
Particles 14056 13831 225 4 447500 1400 4727 0 2883
Insect
Biodetritus 14056 2262 11794 4 4270 0 5 0 0
Fibers 14056 12840 1216 4 24400 52 192 0 112
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Pollen Grains 14056 3276 10780 4 11200 0 9 0 0
Fungal Spores 14056 8816 5240 4 906250
0 8 6239 0 32
Others 14056 13719 337 4 752000 244 1452 0 500
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It is revealed that out of 5,197 air samples collected from
indoor sites, including both residential as well as commercial
buildings, approximately 99.21% (5,156) samples were examined as
positive for skin cells (Photograph 1). Only about 0.79% (41)
samples are negative and devoid of any such particles. The lowest
concentration of skin cells found omitting negative samples is
recorded as 22 counts/m3 in comparison to a maximum value of
171,822 counts/m3. Out
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of seven identified categories of air particulates, skin cells
are noticed as the third most prevalent airborne particle of indoor
environments in regards to average concentration. Out of a total
5,197 air samples collected from indoor sites, Opaque Particles are
reported as the most dominant (5,186), followed by Others (5,163),
Skin Cells (5,156), Fibers (4,720), Fungal Spores (4,615), Pollen
Grains (1,263) and Insect Biodetritus (369) (Figure-1a).
Quantitatively, we have noticed that Opaque Particles dominated
(28,069 counts/m3) over the Others (6,654 counts/m3) followed by
Skin Cells (4,883 counts/m3), Fungal Spores (3,573 counts/m3),
Fibers (309 counts/m3), Pollen Grains (40 counts/m3) and Insect
Biodetritus (4 counts/m3) (Figure-2a). At the 67th percentile of
the collected samples, it is observed that Skin Cells counts (4,940
counts/m3) are lower than Opaque Particles, but higher than the
Other, Fungal Spores, Fibers, Pollen Grains, and Insect Biodetritus
(Table-1).
In the case of surface samples collected from indoor sites,
about 93% (13,073 samples) of samples were examined with skin
cells; however, the number of negative samples (without skin cells)
is close to 7% (983 samples). Data indicates that out of 14,056
surface samples, 13,831 samples are positive for Opaque Particles
followed by Other (13,719), Skin Cells (13,073), Fibers (12,840),
Pollen Grains (3,276) and Insect Biodetritus (2,262) (Figure-1b).
When a comparison is made of the average counts, it stands out that
Fungal Spores (6,239 counts/cm2) dominates over Opaque Particles
(4,727 counts/cm2) followed by Others (1,452 counts/cm2), Skin
Cells (949 counts/cm2), Fibers (192 counts/cm2), Pollen Grains (9
counts/cm2), and Insect Biodetritus (5 counts/cm2) (Figure-2b). We
have also observed that at the 67th percentile, the concentration
of Skin Cells is 356 counts/cm2 which is below the Opaque Particles
and Other, whereas, it is higher than Fibers, Fungal Spores, Pollen
Grains and Insect Biodetritus (Table-2).
The above observations reveal that Skin Cells are one of the
major contributors of bio-components in indoor environments. While
culture-based studies have focused on microbes, such as bacteria,
fungi, etc., as ubiquitous in indoor environments, microscopic
examinations divulge the presence of numerous, non-culturable
particulates in the air and on surfaces. During this study, it is
noticed that although the size range of these particles may vary
from 5 µm to over 1000 µm, they are noticed to typically be within
a size range of 10 µm to 15 µm.
A number of references (Gammage Richard B, 1996; Flannigan et
al. 2001; Scott T. Kelley, et al. 2013) are available to correlate
acute, chronic illness and other ailments which might affect human
health in relation to spending time indoors and away from fresh
air. Numerous indoor air pollutants such as dust mites, mold, pet
dander, environmental tobacco smoke, cockroach allergens,
particulate matter, and others, are “asthma triggers,” meaning that
some asthmatics might experience asthma attacks following exposure
(Institute of
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Medicine 2000). Productivity, comfort and the well-being of
occupants are also compromised due to the diverse nature of indoor
contaminants. It is primarily believed that microbes of indoor
biota may influence healthy living; however, unpleasant odors
originating from other abiogenic and biogenic substances of indoor
sites can distress the comfort and well-being of occupants.
We have observed that these small skin cells may become airborne
and enter into air conveyance systems through return ductwork or
open plenum areas. Once they are airborne, they may start
disseminating through air systems into an entire building. These
particles can also deposit/enter into a buildings air handling
units (AHU), blower cage assembly, porous fiberglass insulation
liner, and subsequently become lodged deep within evaporator coil
fins, also commonly called fouling.
Depending on moisture availability and other environmental
parameters, these tiny particles of indoor environments start
decomposing in that ecological niche, releasing organic matters and
other byproducts. Bacteria and other microbial agents play a
pivotal role in such biodegradation.
A study reveals that human skin flakes lead to “bad smells” in
air-conditioning systems (Lai K.M. et al. 2018). As per the study,
bacteria harbor in air-conditioning systems and utilize human skin
cells as a food source. Ammonia and other volatile fatty acid is
produced due to bacterial enzymatic action, even in a dust free
environment. These compounds release body odors and a urine-like
smell. Odors emitted in air conditioning systems can be easily
distributed within occupiable/air-conditioned areas within the
building, disturb comfort, and raise complaints regarding the
overall indoor air quality.
Contrary to the information above, the presence of human skin
cells can also be helpful in reducing indoor pollution. It is
reported that cholesterol and squalene in house dust helps in
removing ozone (2 to 15%) in an indoor environment (Weschler
Charles J. et al. 2011).
As discussed above, dead human skin cells in indoor environments
have both negative and positive impacts. The dead cells are capable
of removing ozone (one of the significant indoor pollutants in and
around buildings). Ozone is listed as an indoor pollutant that is
capable of reducing lung function and inflaming the linings of the
lungs. Repeated exposure may lead to permanently scarred lung
tissue according to the EPA. As a matter of fact, oil associated
with dead skin cells removes ozone by depleting the three-oxygen
molecule in this molecule. However; the excessive amount of these
entities in indoor environments may influence the building
occupancy by nurturing several other existing bio pollutants of
indoor environments. We have noticed an average concentration of
these particles does not typically exceed 15,000
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counts/m3 in air samples; whereas, not more than 1,200
counts/cm2 in the surface samples. Ameliorating these indoor
pollutants not only helps in improving air quality, but also
provides enhanced productivity and comfort experience of building
occupancy. Air filtration and other therapeutic means, such as
steam cleaning and others, are in practice to reduce the population
of these particulates. Application of HEPA equipped, negative air
machines in indoor environments for 24 to 48 hours significantly
controlled these particles by removing them from air. In modern
buildings, they are transported through the air conveyance system
and deposited on coils (heat exchanger of air conditioning unit)
and thus transform into a reservoir for microbial proliferation and
further distribution in the facility. This activity also interferes
in airflow and restrictive distribution has a direct impact on
energy and contributes to poor indoor quality. Professional steam
treatment (PURE-Steam Coil Cleaning method) is noticed to be 99.99%
effective in removing deposited debris, lint, microbial entities,
skin cells and several other biological as well as a-biological
pollutants to enhance the airflow and optimal health, energy and
comfort quality in building occupancy. The EPA has completed
extensive modeling to assess the compatibilities and tradeoffs
between energy, indoor air quality and thermal comfort that
provides an overall superior performance for building
automation.
At this time, limited data is available to correlate direct
health implications due to skin cells within an indoor environment.
However, skin cell concentrations reported from indoor environments
may be used in combination with other indoor air quality
investigation parameters to determine ventilation adequacy,
occupant density, microbial growth probability, humidity level, and
housekeeping consistency.
The accumulation of skin cells and bacterial degradation of skin
squalene in AHU and associated ductwork can cause a urine-like
smell in air-conditioned systems and is identified as a potential
source of building contaminants.
Odor control in indoor environments is not an easy task. In
day-to-day practice, it is believed that the accretion of dust,
debris, microbes, moisture and other contaminants within
air-conditioning systems is a main source of odor emission;
however, a pronounced “dirty sock” smell is experienced even though
the air-conditioning unit is otherwise free from visible dust
accumulation.
This study reveals that the presence of skin cells in
air-conditioning and other indoor sites is common. Mitigation of
odor control in visibly cleaner environments may be tricky and
requires an adequate building diagnosis,
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inspections, and source determinations through the engineering
and nature of buildings along with the occupant’s behavior.
In order to minimize skin cells and associated building
pollutants in an indoor environment, ecofriendly, green technology
such as high MERV (minimum efficiency reporting value) filtration,
air purification units fitted with HEPA (high efficiency
particulate air) filters (rated at 99.9% efficiency), steam
cleaning of evaporator coils, blower case assemblies and drain pans
as well as topical cleaning practices may be useful techniques in
neutralizing these bio contaminants.
The authors would like to acknowledge all of the past and
present analysts who contributed to the examination of the samples
referred to in this report. Our thanks are also extended to all of
our data entry persons who assisted in insuring that the data was
reported and entered in the most accurate way possible. We would
also like to acknowledge Building Sciences for providing us with
numerous samples from many different locations. We would also like
to thank Pure Air Control Services, Inc., for providing us with the
support and abilities to receive and compile this data in order for
us to create this important document. We would also like to thank
CABA and the Intelligent Buildings Council (IBC) White Paper
Sub-committee. Thank Special thanks to Mrs. Cynthia Bailey, Data
Administrator, for the upkeep and technical changes to Paradox. A
final thanks to anyone else who has helped in one way or another
that lead to us reaching this spectacular achievement.
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